Magnesium alloy and manufacturing method for same

ABSTRACT

There are provided a magnesium alloy material and a method for producing the magnesium alloy material. In a magnesium (Mg) alloy material having a sheet-shaped portion with a thickness of 1.5 mm or more, when a region having ¼ the thickness of the sheet-shaped portion in a thickness direction from a surface of the sheet-shaped portion is defined as a surface region and a remaining region is defined as an internal region, the ratio O F /O C  of the basal plane peak ratio O F  in the surface region to the basal plane peak ratio O C  (degree of orientation of (002) planes) in the internal region satisfies 0.95≦O F /O C ≦1.05. A sheet-shaped Mg alloy material is obtained by performing at least one pass of the rolling at a reduction ratio of 25% or more and the remaining passes of the rolling at a reduction ratio of 10% or more.

TECHNICAL FIELD

The present invention relates to a magnesium alloy material that is suitable for various members such as parts of transport machines, e.g., automobiles, railway vehicles, and airplanes, parts of bicycles, housings of electric and electronic devices, and other structural members and that is also suitable for constituent materials for the members, and to a method for producing the magnesium alloy material. In particular, the present invention relates to a thick magnesium alloy material having good plastic formability such as press formability.

BACKGROUND ART

Lightweight magnesium alloys having high specific strength and specific rigidity have been investigated as constituent materials for various members such as housings of mobile electric and electronic devices, e.g., cellular phones and laptop computers, parts of automobiles, e.g., wheel covers and paddle shifts, parts of railway vehicles, and parts of bicycles, e.g., frames. Members composed of a magnesium alloy are mainly formed of cast materials (AZ91 alloy of the American Society for Testing and Materials (ASTM) standard) by a die casting process or a thixomolding process. In recent years, press-formed materials obtained by performing press forming on a sheet composed of a wrought magnesium alloy such as AZ31 alloy of the ASTM standard have been used. Patent Literature 1 discloses that a continuous cast material composed of a magnesium alloy such as AZ91 alloy is produced by a twin-roll casting process, and a rolled sheet obtained by rolling the continuous cast material is subjected to press forming

CITATION LIST Patent Literature

-   Patent Literature 1: International Publication No. 2006/003899

SUMMARY OF INVENTION Technical Problem

Focusing on the lightweight of magnesium alloys, a relatively thin sheet having a thickness of 1 mm or less has been investigated as a raw material for plastic formed materials such as press formed materials. However, with the expansion of usage of magnesium alloys, the development of not only the above-described thin sheet but also a thick sheet, specifically, a thick sheet having a thickness of 1.5 mm or more has been demanded focusing on the specific strength and specific rigidity. Such a thick magnesium alloy sheet having good plastic formability, a method for producing the thick magnesium alloy sheet, and a plastic formed material such as a press formed material produced using the thick magnesium alloy sheet have not been sufficiently studied.

A thick magnesium alloy sheet is obtained by employing a die casting process or a thixomolding process. However, in cast materials such as die cast materials, internal defects such as cavities are easily formed, and furthermore the composition and structure tend to become uneven. For example, additive element components are locally highly concentrated and crystal grains are randomly oriented. Also in cast materials such as die cast materials, precipitates are easily formed in crystal grain boundaries. Since fracture is caused from, for example, the defects and the precipitates in the crystal grain boundaries, cast materials such as die cast materials have poor plastic formability such as press formability. Furthermore, cast materials such as die cast materials have poor mechanical properties such as strength and hardness compared with plastic formed materials such as press formed materials due to the above internal defects or the like.

Accordingly, it is an object of the present invention to provide a thick magnesium alloy material having good plastic formability and a thick magnesium alloy material subjected to plastic forming. It is another object of the present invention to provide a method for producing a thick magnesium alloy material having good plastic formability.

Solution to Problem

A magnesium alloy material subjected to plastic forming (primary forming) such as rolling is excellent in terms of mechanical properties such as strength, hardness, and toughness, corrosion resistance, and plastic formability compared with die cast materials and thixomolded materials even if they have the same composition. This is because, in such a magnesium alloy material, formation of defects during casting is reduced and the crystal is made finer. A magnesium alloy material obtained by performing plastic forming (secondary forming) such as press forming on the magnesium alloy material subjected to the primary forming is also excellent in terms of mechanical properties and corrosion resistance. In particular, when a continuous cast material produced by a continuous casting process such as a twin-roll casting process is used as a raw material for primary formed materials, the continuous cast material has good plastic formability because segregation and generation of coarse impurities in crystal and precipitated impurities are reduced compared with die cast materials. Therefore, the inventors of the present invention have produced a thick magnesium alloy sheet having a thickness of 1.5 mm or more by rolling a continuous cast material under various conditions and have measured the plastic formability. Herein, a rolled material (rolled sheet) composed of a magnesium alloy generally has a texture in which basal planes of magnesium alloy crystals are oriented so as to be parallel to the rolling direction (a direction in which a raw material to be rolled travels). If the degree of orientation density of the texture is high, there is a drawback of impairing the formability during plastic forming such as press forming. The inventors of the present invention have found the following two points: (1) when the degree of orientation density of a texture in a surface-side portion of a rolled sheet is higher than that of a texture in an internal portion of the rolled sheet, the formability (plastic formability) considerably degrades; and (2) if a thick rolled material is obtained by performing a known rolling process on a thick raw material, the texture in the surface-side portion is developed compared with the texture in the internal portion, and thus the formability substantially equivalent to that of a thin sheet having a thickness of 1.5 mm or less is not achieved. Furthermore, the inventors of the present invention have found that a magnesium alloy sheet produced under certain conditions has good plastic formability such as press formability while being thick. The present invention is based on the above findings.

A magnesium alloy material of the present invention is composed of a magnesium alloy and includes a sheet-shaped portion having a thickness of 1.5 mm or more. The sheet-shaped portion satisfies the following orientation,

[Orientation]

when a region having ¼ the thickness of the sheet-shaped portion in a thickness direction from a surface of the sheet-shaped portion is defined as a surface region and a remaining region is defined as an internal region,

X-ray diffraction peak intensities of a (002) plane, a (100) plane, a (101) plane, a (102) plane, a (110) plane, and a (103) plane in the surface region are respectively defined as I_(F)(002), I_(F)(100), I_(F)(101), I_(F)(102), I_(F)(110), and I_(F)(103),

X-ray diffraction peak intensities of a (002) plane, a (100) plane, a (101) plane, a (102) plane, a (110) plane, and a (103) plane in the internal region are respectively defined as I_(C)(002), I_(C)(100), I_(C)(101), I_(C)(102), I_(C)(110), and I_(C)(103), a degree of orientation of the (002) plane in the surface region, which is expressed as I_(F)(002)/{I_(F)(100)+I_(F)(002)+I_(F)(101)+I_(F)(102)+I_(F)(110)+I_(F)(103)}, is defined as a basal plane peak ratio O_(F), and

a degree of orientation of the (002) plane in the internal region, which is expressed as I_(C)(002)/{I_(C)(100)+I_(C)(002)+I_(C)(101)+I_(C)(102)+I_(C)(110)+I_(C)(103)}, is defined as a basal plane peak ratio O_(C),

a ratio O_(F)/O_(C) of the basal plane peak ratio O_(F) in the surface region to the basal plane peak ratio O_(C) in the internal region satisfies 0.95≦O_(F)/O_(C)≦1.05.

The magnesium alloy material of the present invention can be produced, for example, by the following production method of the present invention. A method for producing a magnesium alloy material according to the present invention is a method for producing a magnesium alloy material by rolling a raw material composed of a magnesium alloy, the method including a preparation step and a rolling step below.

Preparation step: a step of preparing a sheet-shaped raw material obtained by subjecting a molten magnesium alloy to continuous casting by a twin-roll casting process

Rolling step: a step of rolling the raw material with multiple passes to produce a sheet-shaped magnesium alloy material having a thickness of 1.5 mm or more

In the rolling step, at least one pass of the rolling is performed at a reduction ratio of 25% or more per pass and the remaining passes of the rolling are performed at a reduction ratio of 10% or more per pass.

The reduction ratio (%) herein is defined as {(thickness t_(b) of raw material before rolling−thickness t_(a) of raw material after rolling)/thickness t_(b) of raw material before rolling}×100.

According to the production method of the present invention, relatively severe rolling at a reduction ratio of 25% or more per pass can be favorably performed by employing, as a raw material, a continuous cast material in which defects and impurities in crystal and precipitated impurities, from which cracking is caused, and segregation are present in a small amount or are substantially not present. In this rolling at a high reduction ratio, plastic forming can be uniformly imparted across the entire region in the thickness direction of the raw material. In other words, by performing at least one pass of the rolling at a high reduction ratio, the raw material can be uniformly subjected to forming from the surface to the inside of the raw material. Therefore, the production method of the present invention provides a magnesium alloy material (typically a rolled sheet (one form of the magnesium alloy material of the present invention)) constituted by a homogeneous structure across the entire region in the thickness direction. This structure is a texture in which basal planes of magnesium alloy crystals are mainly oriented so as to be parallel to the rolling direction (a texture in which c axes of the magnesium alloy crystals are oriented so as to be orthogonal to the rolling direction).

When the magnesium alloy material of the present invention is a rolled sheet subjected to the above-described particular rolling (i.e., when the entire magnesium alloy material of the present invention is constituted by the sheet-shaped portion), the magnesium alloy material is constituted by a homogeneous structure across the entire region (surface˜center˜surface) in the thickness direction thereof as described above. The magnesium alloy material of the present invention constituted by such a homogeneous structure has good plastic formability such as press formability while being thick. Thus, the sheet-shaped magnesium alloy material can be suitably used as a raw material for plastic forming such as press forming. The magnesium alloy material constituted by such a homogeneous structure also has uniform properties (e.g., mechanical properties such as hardness, strength, impact resistance, and toughness, corrosion resistance, and vibration resistance). Furthermore, when the sheet-shaped magnesium alloy material is used as a raw material for plastic forming, a plastic formed material (one form of the magnesium alloy material of the present invention) such as a press formed material having high dimensional accuracy is obtained. The obtained plastic formed material is also constituted by a homogeneous structure across the entire region in the thickness direction thereof, that is, the structure of the raw material is substantially maintained. Accordingly, the obtained plastic formed material such as a press formed material also has the above-described uniform properties.

According to an embodiment of the magnesium alloy material of the present invention, when an average crystal grain size in the surface region is defined as D_(F) and an average crystal grain size in the internal region is defined as D_(C), a ratio D_(C)/D_(F) of the average crystal grain size D_(C) in the internal region to the average crystal grain size D_(F) in the surface region satisfies ⅔≦D_(C)/D_(F)≦3/2, D_(F)≦3.5 μm), and D_(C)≧3.5 μm.

According to the above embodiment, since a uniform grain size is achieved across the entire region in the thickness direction, better plastic formability is imparted.

According to an embodiment of the magnesium alloy material of the present invention, when a Vickers hardness (Hv) in the surface region is defined as H_(F) and a Vickers hardness (Hv) in the internal region is defined as H_(C), a ratio H_(C)/H_(F) of the Vickers hardness H_(C) in the internal region to the Vickers hardness H_(F) in the surface region satisfies 0.85≦H_(C)/H_(F)≦1.2.

According to the above embodiment, the magnesium alloy material has a uniform hardness across the entire region in the thickness direction. Therefore, for example, even when part of the magnesium alloy material is subjected to grinding or a chemical treatment to remove part of the surface-side portion, the surface hardness of the magnesium alloy material subjected to the grinding or the chemical treatment substantially does not change compared with before the grinding or the chemical treatment, which means that the magnesium alloy material has a stable surface texture. Thus, according to the above embodiment, when a surface treatment such as a chemical conversion treatment is performed in downstream processes, such a treatment can be stably performed.

The magnesium alloy material of the present invention can be composed of a magnesium alloy containing various additive elements (balance: Mg and impurities). In particular, an alloy containing additive elements in a high concentration, specifically, a magnesium alloy containing additive elements in a total content of 5.0 mass % or more has good mechanical properties such as strength and hardness, high corrosion resistance, flame resistance, heat resistance, and the like, though depending on the types of additive elements.

Specific examples of the additive elements include at least one element selected from Al, Zn, Mn, Si, Be, Ca, Sr, Y, Cu, Ag, Sn, Li, Zr, Ce, Ni, Au, and rare-earth elements (except for Y and Ce). An example of the impurities is Fe.

A Mg—Al series alloy containing Al has high corrosion resistance and also has good mechanical properties such as strength and hardness. Therefore, according to an embodiment of the magnesium alloy material of the present invention, the magnesium alloy contains Al as an additive element in a content of 5.0 mass % or more and 12 mass % or less. As the Al content increases, the above effects tend to increase. The Al content is preferably 7 mass % or more and more preferably 7.3 mass % or more. The upper limit of the Al content is 12 mass % and preferably 11 mass % because the plastic formability degrades when the Al content exceeds 12 mass %. In particular, a magnesium alloy containing 8.3 mass % to 9.5 mass % of Al provides high strength and corrosion resistance. The total content of elements other than Al is 0.01 mass % or more and 10 mass % or less and preferably 0.1 mass % or more and 5 mass % or less.

Specific examples of the Mg—Al series alloy include AZ series alloys (Mg—Al—Zn series alloys, Zn: 0.2 mass % to 1.5 mass %, such as AZ 31 alloy, AZ61 alloy, and AZ91 alloy) in the ASTM standard, AM series alloys (Mg—Al—Mn series alloys, Mn: 0.15 mass % to 0.5 mass %), AS series alloys (Mg—Al—Si series alloys, Si: 0.01 mass % to 20 mass %), Mg—Al-RE (rare-earth element) series alloys, AX series alloys (Mg—Al—Ca series alloys, Ca: 0.2 mass % to 6.0 mass %), and AJ series alloys (Mg—Al—Sr series alloys, Sr: 0.2 mass % to 7.0 mass %). Examples of the alloy containing 8.3 mass % to 9.5 mass % of Al include Mg—Al—Zn series alloys further containing 0.5 mass % to 1.5 mass % of Zn, e.g., AZ91 alloy.

A magnesium alloy containing at least one element selected from Y, Ce, Ca, Si, Sn, and rare-earth elements (except for Y and Ce) in a total content of 0.001 mass % or more and preferably 0.1 mass % or more and 5 mass % or less, the balance being Mg and impurities, provides high heat resistance and flame resistance. When rare-earth elements are contained, the total content is preferably 0.1 mass % or more. In particular, when Y is contained, the content is preferably 0.5 mass % or more.

Advantageous Effects of Invention

A magnesium alloy material according to the present invention is a thick magnesium alloy material having good plastic formability. In a method for producing a magnesium alloy material according to the present invention, a thick magnesium alloy material having good plastic formability can be produced.

DESCRIPTION OF EMBODIMENTS

The present invention will now be further described in detail.

[Magnesium Alloy Material] (Composition)

The magnesium alloy material of the present invention is composed of a magnesium alloy containing 50 mass % or more of Mg and, typically, the additive elements described above.

(Form)

A sheet-shaped portion in the magnesium alloy material of the present invention means a portion having a pair of surfaces parallel to each other with a substantially uniform interval (distance) between the surfaces, that is, a portion having a uniform thickness. The magnesium alloy material of the present invention has a sheet-shaped portion in at least part thereof. When this is satisfied, the magnesium alloy material may have a form having a portion with a thickness locally different from that of the other portions through a process such as a cutting process, e.g., a form including, for example, a boss joined thereto, a form having a groove, and a form having a through-hole that connects the top and the bottom.

A typical form of the magnesium alloy material of the present invention having the sheet-shaped portion is a form (magnesium alloy sheet) in which the entire magnesium alloy material has a sheet-like shape. The shape (planar shape) of the magnesium alloy sheet may be a rectangular shape, a circular shape, or the like. The magnesium alloy sheet may be in the form of a coil stock obtained by coiling a continuous long sheet or in the form of a short sheet having a predetermined length and shape. The magnesium alloy sheet may have various forms in accordance with the production process. Examples of the forms include a rolled sheet, a heat treated sheet and a leveled sheet respectively obtained by performing a heat treatment and leveling described below on the rolled sheet, and a polished sheet and a coated sheet respectively obtained by performing polishing and coating on the rolled sheet, heat-treated sheet, or leveled sheet.

The magnesium alloy material of the present invention may also be a formed product obtained by performing plastic forming (secondary forming) such as press forming, e.g., bending and drawing on the magnesium alloy sheet or a partly formed material having a plastic formed portion, which is obtained by partly performing plastic forming on the magnesium alloy sheet (herein, at least part of the formed product or partly formed material is the sheet-shaped portion). The formed product may be a box or frame having a U-shaped cross section that includes a top (bottom) and a sidewall extending perpendicularly from the periphery of the top (bottom) or a covered tube that includes a discoidal top and a cylindrical sidewall. At least the top corresponds to the sheet-shaped portion. The form of the magnesium alloy material can be selected in accordance with desired applications.

(Thickness)

In the magnesium alloy material of the present invention, the sheet-shaped portion has a thickness of 1.5 mm or more. Any thickness can be selected from a thickness of 1.5 mm or more in accordance with desired applications. Herein, to increase the thickness of the sheet-shaped portion, the thickness of a cast material serving as a raw material needs to be also increased. If the thickness of the cast material is increased, the rollability is degraded due to the above-described defects or the like. Therefore, the thickness of the sheet-shaped portion is preferably 10 mm or less and particularly preferably 5 mm or less for the purpose of producing a thick rolled sheet (one form of the magnesium alloy material of the present invention) with high productivity.

When the magnesium alloy material of the present invention is the formed product or the partly formed material, the structure and mechanical properties of the magnesium alloy sheet serving as a raw material for plastic forming are substantially maintained in a portion (e.g., the sheet-shaped portion) that undergoes less deformation during the plastic forming

(Structure) <Orientation>

In the magnesium alloy material of the present invention, at least the sheet-shaped portion is constituted by a structure having a homogeneous texture across the entire sheet-shaped portion in the thickness direction. A form in which the ratio O_(F)/O_(C) of basal plane peak ratios particularly satisfies 1.00≦O_(F)/O_(C)≦1.05 has better plastic formability. A plastic formed material (formed product) or a partly formed material has a small variation in mechanical properties across the entirety thereof

<Average Crystal Grain Size>

A typical form of the magnesium alloy material of the present invention is a form in which the crystal grain size is uniform across the entire region in the thickness direction as described above. In this form, when plastic forming such as press forming is performed, uniform deformation can be achieved and thus a plastic formed material (formed product) or a partly formed material having high dimensional accuracy is obtained. Plastic forming is expected to be uniformly performed as the difference in the average crystal grain size between the surface region and the internal region becomes smaller. Therefore, a form in which the ratio D_(C)/D_(F) of average crystal grain sizes particularly satisfies 1≦D_(C)/D_(F)≦1.4 has better plastic formability. A plastic formed material (formed product) or a partly formed material has a small variation in mechanical properties across the entirety thereof

In the case where a thick sheet-shaped magnesium alloy material having a thickness of 1.5 mm or more is produced by performing rolling as described above, there are limitations to achievement of a uniform and fine grain size across the entire region in the thickness direction. The minimum average crystal grain size is about 3.5 μm. However, since the plastic formability tends to improve as the crystal grain size decreases, the average crystal grain size of the sheet-shaped portion is preferably 20 μm or less and particularly preferably 10 μm or less. The average crystal grain size varies depending on the reduction ratio and the heating temperature of a raw material in the rolling step, and tends to decrease as the reduction ratio increases and the heating temperature decreases. The average crystal grain size in the surface region in which plastic forming is sufficiently imparted during rolling tends to be smaller than that in the internal region.

(Mechanical Properties)

The magnesium alloy material of the present invention has better mechanical properties such as strength, hardness, and toughness than cast materials such as die cast materials and also has uniform mechanical properties across the entire region in the thickness direction thereof because the magnesium alloy material is subjected to rolling. For example, the magnesium alloy material of the present invention has a uniform Vickers hardness as described above. When the difference in Vickers hardness between the surface region and the internal region is small, a mechanically weak spot (low hardness spot) is substantially not present. Therefore, the ratio H_(C)/H_(F) of Vickers hardnesses (Hv) more preferably satisfies 0.95≦H_(C)/H_(F)≦1.1. The absolute value of the Vickers hardness tends to increase as the content of additive elements increases, though depending on the rolling conditions such as the reduction ratio and the heating temperature of a raw material. The Vickers hardness in the surface region in which plastic forming is sufficiently imparted during rolling tends to be higher than that in the internal region. When the magnesium alloy material of the present invention is a plastic formed material (formed product) or a partly formed material, the hardness tends to be further increased by work hardening.

(Other Structures)

By subjecting at least part of the surface of the magnesium alloy material of the present invention to an anti-corrosion treatment such as a chemical conversion treatment or an anodic oxidation treatment to form an anti-corrosion layer, higher corrosion resistance is achieved. Furthermore, by coating at least part of the surface of the magnesium alloy material of the present invention to form a coating layer, the design and commercial value are improved.

[Production Method]

Each step of the above-described production method of the present invention will now be further described in detail.

(Preparation Step) <Casting>

In the production method of the present invention, a continuous cast material is used as a starting material. In a continuous casting process, rapid solidification can be performed. Therefore, segregation and formation of oxides can be reduced even when additive elements are contained in a large amount, which can suppress the generation of coarse impurities in crystal and precipitated impurities having a size of more than 10 μm from which cracking may be caused. Thus, a cast material having good plastic formability such as rollability can be produced. In a continuous casting process, a long cast material can also be produced in a continuous manner. A long material produced by the continuous casting process can be used as a raw material for rolling. When a long raw material is used, a long rolled material can be produced. Examples of the continuous casting process include a twin-roll process, a twin-belt process, and a belt-and-wheel process. A twin-roll process or a twin-belt process is suitable for the production of sheet-shaped cast materials, and a twin-roll process is particularly suitable. A continuous cast material produced by the casting process described in Patent Literature 1 is preferably used. The thickness, width, and length of a cast material can be suitably selected so that a desired rolled material (rolled sheet) is obtained. The thickness of the cast material is preferably 10 mm or less and particularly preferably 5 mm or less because segregation is easily caused in an excessively thick cast material. When the obtained continuous cast material is used as a long material, such a continuous cast material is coiled in a cylindrical shape because the continuous cast material is easily transferred to the next step. When the cast material is coiled while a start-of-coiling portion of the cast material is heated to about 100° C. to 200° C., even alloys such as AZ91 alloy which contain additive elements in a large amount and easily cause cracking are easily bent. Even in the case of a small coiling diameter, the cast material can be coiled without being cracked. The obtained continuous cast material can be cut into sheet materials having a desired length to obtain a raw material for rolling. In this case, a rolled material (rolled sheet) having a desired length is obtained.

<Solution Treatment>

By performing a solution treatment before the cast material is rolled, the composition of the cast material can be homogenized and elements such as Al can be sufficiently dissolved to improve the toughness. The solution treatment is performed at a heating temperature of 350° C. or more and particularly 380° C. or more and 420° C. or less for a holding time of 1 hour or more and 40 hours or less. In the case of Mg—Al series alloys, the holding time is preferably increased as the content of Al increases. In a cooling step from the heating temperature after the holding time, the precipitation of coarse precipitates can be suppressed by increasing the cooling rate (preferably 50° C./min or more) using accelerated cooling such as water cooling or air blast cooling.

<Rolling>

The cast material or the material subjected to a solution treatment is rolled with multiple passes. At least one pass is preferably performed by warm rolling which is performed while a raw material (a cast material, a material subjected to a solution treatment, or a worked material being subjected to rolling) is heated to 150° C. or more and 400° C. or less or by hot rolling. By heating the raw material to the above temperature, cracking or the like is not easily caused during the rolling even if the reduction ratio per pass is increased. As the temperature is increased, the formation of cracks is reduced. By setting the temperature to be 400° C. or less, the degradation caused by seizing of a surface of the raw material and the thermal degradation of a reduction roll can be suppressed. Therefore, the heating temperature is preferably 350° C. or less, more preferably 300° C. or less, and particularly preferably 150° C. or more and 280° C. or less. In addition to the raw material, the reduction roll may also be heated. The heating temperature of the reduction roll is, for example, 100° C. to 250° C.

In the production method of the present invention, the rolling is performed with one pass or multiple passes at a reduction ratio of 25% or more per pass (hereinafter this rolling is referred to as severe rolling). Such severe rolling at a high reduction ratio is preferably the above-described warm rolling or hot rolling. At a higher reduction ratio, plastic forming can be sufficiently imparted to a raw material from the surface to the inside and a rolled material constituted by a homogeneous structure is obtained. Therefore, the reduction ratio of the severe rolling is preferably 30% or more per pass and can be suitably selected in the range in which no cracking is caused on a raw material. When rolling other than the severe rolling (hereinafter this rolling is referred to as general rolling) is performed at a reduction ratio of 10% or more per pass, the rolling can be uniformly and sufficiently performed across the entire region in the thickness direction of the raw material. As described above, since plastic forming can be sufficiently imparted as the reduction ratio of each pass of the general rolling increases, the reduction ratio per pass may be set to be 15% or more and furthermore 20% or more. The number of passes and reduction ratio of the severe rolling and general rolling can be suitably selected in accordance with the total reduction ratio.

The conditions such as the heating temperature of a raw material, the temperature of a reduction roll, and the reduction ratio may be changed for each pass. Therefore, the reduction ratio of each pass may be the same or different. An intermediate heat treatment may be performed between the passes. By performing the intermediate heat treatment, the strain and residual stress introduced into the raw material before the intermediate heat treatment can be removed or reduced to allow ease of rolling after the intermediate heat treatment. The intermediate heat treatment can be performed at a heating temperature of 150° C. to 350° C. (preferably 300° C. or less, more preferably 250° C. to 280° C.) for a holding time of 0.5 to 3 hours. After the rolling, a final heat treatment may be performed under the above conditions. Furthermore, the rolling is easily performed by suitably using a lubricant because the frictional resistance during rolling can be reduced and the seizing of the raw material can be prevented.

In addition, the edge of the cast material before rolling may be trimmed to prevent the extension of cracks, which may present at the edge, during rolling. Such trimming may be performed to appropriately adjust the width during rolling or after rolling.

<Other Processes> <<Polishing>>

After the rolling, polishing may be performed. The polishing is performed in order to remove or reduce the lubricant used during the rolling and the scratches and oxide films present on the surface of the rolled material. The polishing is preferably performed with a grinding belt because even a long material can be easily polished in a continuous manner. The polishing is preferably performed by a wet process to prevent scattering of powder.

<<Leveling>>

After the rolling or after the polishing, leveling may be performed. The leveling is performed in order to improve the flatness and precisely perform plastic forming such as press forming. In the leveling, a roll leveler including a plurality of rollers disposed in a staggered manner can be suitably used. The leveling may be performed while the raw material is heated to, for example, 100° C. to 300° C. and particularly 150° C. to 280° C. (warm leveling).

<<Plastic Forming>>

When the magnesium alloy material of the present invention is processed into a formed product or a partly formed material having a plastic formed portion, such a formed product or a partly formed material can be produced by a production method that includes a plastic forming step of performing plastic forming such as press forming on at least part of the raw material (the above-described rolled material, polished material, or leveled material) subjected to the above rolling step. The plastic forming is preferably performed in the temperature range of 200° C. to 300° C. to improve the plastic formability of the raw material. A heat treatment may be performed after the plastic forming in order to remove the strain and residual stress introduced during the plastic forming and improve the mechanical properties. The heat treatment is performed at a heating temperature of 100° C. to 300° C. for a heating time of about 5 to 60 minutes.

<<Surface Treatment>>

When the magnesium alloy material of the present invention includes the above-described anti-corrosion layer or coating layer, the production can be performed by a production method that includes a surface treating step of performing an anti-corrosion treatment or coating on at least part of the raw material subjected to the rolling step or at least part of the raw material subjected to the plastic forming step. Furthermore, at least one selected from hairline finish, diamond cutting, shot blasting, etching, and spin cutting may be performed on at least part of the raw material. Such a surface treatment is performed in order to improve the corrosion resistance and a mechanical protective function and improve the design, metal texture, and commercial value.

A specific embodiment of the present invention will now be further described based on Test Examples.

[Test Example]

Raw materials composed of magnesium alloys having the following compositions were subjected to rolling under various conditions to produce magnesium alloy sheets having a thickness of 1.5 mm or more. The orientation, crystal grain size, and Vickers hardness of the magnesium alloy sheets were measured.

In this test, a magnesium alloy sheet composed of a magnesium alloy (Mg-8.9 mass % Al-0.6 mass % Zn) having a composition equivalent to that of AZ91 alloy and a magnesium alloy sheet composed of a magnesium alloy (Mg-3.0 mass % Al-0.7 mass % Zn) having a composition equivalent to that of AZ31 alloy were produced.

Long cast sheets (thickness 4.5 mm (4.50 to 4.51 mm)×width 320 mm) were produced from the above magnesium alloys having the above compositions by a twin-roll continuous casting process. The long cast sheets were temporarily coiled to produce cast coil stocks. Each of the cast coil stocks was subjected to a solution treatment at 400° C. for 24 hours. A raw material obtained by uncoiling the solid solution coil stock subjected to the solution treatment was rolled with multiple passes under the rolling conditions shown in Table Ito produce a rolled material (magnesium alloy sheet) having a thickness of 2.0 mm (2.00 mm to 2.01 mm) or 1.5 mm (1.50 mm to 1.51 mm). Each pass was performed by warm rolling (the heating temperature of a raw material: 250° C. to 280° C., the temperature of a reduction roll: 100° C. to 250° C.). The thickness of the cast material, the thickness of a worked material being subjected to rolling, and the thickness of the obtained magnesium alloy sheet were determined to be the average of three thicknesses in total at the central part in the width direction of a sheet to be measured and two points located 50 mm from the edges in the width direction.

TABLE I Thickness of each pass (mm) Sample Before Reduction ratio of each pass (%) Composition No. rolling 1 2 3 4 5 1 2 3 4 5 AZ91 A 4.50 3.02 2.01 — — — 32.9 33.4 — — — B 4.51 2.99 2.45 2.01 — — 33.7 18.1 18.0 — — C 4.51 3.46 2.88 2.40 2.00 — 23.3 16.8 16.7 16.7 — D 4.50 3.00 2.11 1.51 — — 33.3 29.7 28.4 — — E 4.51 3.20 2.30 1.85 1.50 — 29.0 28.1 19.6 18.9 — F 4.51 3.51 2.77 2.20 1.82 1.51 22.2 21.1 20.6 17.3 17.0 AZ31 G 4.51 3.01 2.01 — — — 33.3 33.2 — — — H 4.50 2.98 2.44 2.01 — — 33.8 18.1 17.6 — — I 4.51 3.45 2.89 2.39 2.01 — 23.5 16.2 17.3 15.9 — J 4.51 2.99 2.10 1.51 — — 33.7 29.8 28.1 — — K 4.50 3.19 2.29 1.86 1.51 — 29.1 28.2 18.8 18.8 — L 4.51 3.50 2.78 2.19 1.83 1.51 22.4 20.6 21.2 16.4 17.5

[Orientation]

Each of the obtained magnesium alloy sheets was analyzed by X-ray diffraction to determine the ratio O_(F)/O_(C) of the basal plane peak ratio O_(F) in the surface region to the basal plane peak ratio O_(C) in the internal region. Table II shows the results. The basal plane peak ratio O_(F) in the surface region was measured by performing X-ray diffraction on the surface of the magnesium alloy sheet. The basal plane peak ratio O_(C) in the internal region was measured by chemically removing a region (surface region) having ¼ the thickness of the magnesium alloy sheet in a thickness direction from a surface of the magnesium alloy sheet to expose the inside portion and then performing X-ray diffraction on the exposed surface. The peak intensities of a (002) plane, a (100) plane, a (101) plane, a (102) plane, a (110) plane, and a (103) plane in each of the regions were measured to determine the ratio O_(F)/O_(C).

Basal plane peak ratio O_(F):I_(F)(002)/{I_(F)(100)+I_(F)(002)+I_(F)(101)+I_(F)(102)+I_(F)(110)+I_(F)(103)}

Basal plane peak ratio O_(C):I_(C)(002)/{I_(C)(100)+I_(C)(002)+I_(C)(101)+I_(C)(102)+I_(C)(110) I_(C)(103)}

[Average Crystal Grain Size]

The average crystal grain sizes (μm) in the internal region and surface region of each of the obtained magnesium alloy sheets were measured in conformity with “Steels—Micrographic determination of the apparent grain size JIS G 0551 (2005)”. Sections (cross section and longitudinal section) of the magnesium alloy sheet in the thickness direction were taken and observed with an optical microscope (400 times) to determine the average crystal grain size in each of three fields of view (the total number of fields of view in each region: 6) in the sections of the surface region (a region having ¼ the thickness from a surface in the thickness direction) and internal region (a remaining region obtained by removing the surface region). Table II shows the average (D_(F)) of the average crystal grain sizes in the six fields of view in total in the surface region and the average (D_(C)) of the average crystal grain sizes in the six fields of view in total in the internal region. The ratio D_(C)/D_(F) of the average crystal grain size D_(C) in the internal region to the average crystal grain size D_(F) in the surface region was also determined Table II shows the results.

[Vickers Hardness]

The Vickers hardnesses (Hv) in the internal region and surface region of each of the obtained magnesium alloy sheets were measured. As in the case of the measurement of the average crystal grain size, sections (cross section and longitudinal section) of the magnesium alloy sheet in the thickness direction were taken. The Vickers hardness H_(F) in the surface region was measured by pressing an indenter against each of the sections of the surface region. The Vickers hardness H_(F) in the internal region was measured by pressing an indenter against each of the sections of the internal region. Table II shows the average (H_(F)) of the Vickers hardnesses in the sections of the surface region and the average (H_(C)) of the Vickers hardnesses in the sections of the internal region. The ratio H_(C)/H_(F) of the Vickers hardness H_(C) in the internal region to the Vickers hardness H_(F) in the surface region was also determined. Table II shows the results.

TABLE II Basal plane peak Crystal grain size Vickers hardness ratio (μm) (Hv) Sample Surface Internal Surface/Internal Surface Internal Internal/Surface Surface Internal Internal/Surface Composition No. O_(F) O_(C) O_(F)/O_(C) D_(F) D_(C) D_(C)/D_(F) H_(F) H_(C) H_(C)/H_(F) AZ91 A 0.860 0.840 1.02 5.2 5.3 1.02 89 88 0.99 B 0.872 0.832 1.05 5.9 7.2 1.22 88 75 0.85 C 0.869 0.813 1.07 5.2 8.3 1.60 89 73 0.82 D 0.878 0.860 1.02 4.9 5.1 1.04 90 89 0.99 E 0.879 0.843 1.04 5 6.8 1.36 89 79 0.89 F 0.882 0.814 1.08 5.2 8.2 1.58 90 75 0.83 AZ31 G 0.710 0.690 1.03 6.1 6.5 1.07 67 65 0.97 H 0.718 0.685 1.05 5.7 7.8 1.37 69 60 0.87 I 0.717 0.656 1.08 4.9 8.9 1.82 70 58 0.83 J 0.724 0.715 1.01 6.1 6.5 1.07 67 65 0.97 K 0.725 0.699 1.04 5.7 7.8 1.37 69 60 0.87 L 0.720 0.675 1.07 4.9 8.9 1.82 70 58 0.83

As is clear from Tables I and II, by performing both rolling with one pass or more at a reduction ratio of 25% or more per pass and rolling at a reduction ratio of 10% or more per pass on a continuous cast material, there is provided a thick magnesium alloy sheet (magnesium alloy material) which has a thickness of 1.5 mm or more and is constituted by a homogeneous structure in the thickness direction, the structure having a particular orientation. It is also clear that the magnesium alloy sheet has uniform mechanical properties.

Each of the obtained magnesium alloy sheets was subjected to press forming. The press forming was performed by setting the heating temperature of the magnesium alloy sheet to be 250° C. to 270° C. As a result, sample Nos. A, B, D, E, G, H, J, and K constituted by a homogeneous structure in the thickness direction, the structure having a particular orientation, had good press formability and high dimensional accuracy. In the measurement of the structure of a flat portion in the sample Nos. A, B, D, E, G, H, J, and K, the structure of a flat portion was substantially the same as that of the magnesium alloy sheet before press forming, and the samples had the above-described orientation and average crystal grain size.

It was confirmed from the above test results that a magnesium alloy material which has a thick sheet-shaped portion with a thickness of 1.5 mm or more and in which the sheet-shaped portion is constituted by a homogeneous structure in the thickness direction, the structure having a particular orientation, had good press formability. It was also confirmed that the magnesium alloy material subjected to press forming was constituted by a homogeneous structure.

The above-described embodiments can be suitably modified without departing from the scope of the present invention and are not limited to the above configurations. For example, the composition of the magnesium alloy, the thickness and shape of the magnesium alloy material, and the reduction ratio of each pass and the number of passes in the rolling step can be suitably changed.

INDUSTRIAL APPLICABILITY

The magnesium alloy material of the present invention can be suitably used for members in various fields, such as parts of automobiles, parts of railway vehicles, parts of airplanes, parts of bicycles, and parts of electric and electronic devices; constituent materials for the members; and bags and raw materials thereof. The method for producing a magnesium alloy material according to the present invention can be suitably used for the production of the above magnesium alloy material of the present invention. 

1.-5. (canceled)
 6. A magnesium alloy material composed of a magnesium alloy, comprising a sheet-shaped portion, wherein the sheet-shaped portion has a thickness of 1.5 mm or more, and the sheet-shaped portion satisfies the following orientation, [Orientation] when a region having ¼ the thickness of the sheet-shaped portion in a thickness direction from a surface of the sheet-shaped portion is defined as a surface region and a remaining region is defined as an internal region, X-ray diffraction peak intensities of a (002) plane, a (100) plane, a (101) plane, a (102) plane, a (110) plane, and a (103) plane in the surface region are respectively defined as I_(F)(002), I_(F)(100), I_(F)(101), I_(F)(102), I_(F)(110), and I_(F)(103), X-ray diffraction peak intensities of a (002) plane, a (100) plane, a (101) plane, a (102) plane, a (110) plane, and a (103) plane in the internal region are respectively defined as I_(C)(002), I_(C)(100), I_(C)(101), I_(C)(102), I_(C)(110), and I_(C)(103), a degree of orientation of the (002) plane in the surface region, which is expressed as I_(F)(002)/{I_(F)(100)+I_(F)(002)+I_(F)(101)+I_(F)(102)+I_(F)(110)+I_(F)(103)}, is defined as a basal plane peak ratio O_(F), and a degree of orientation of the (002) plane in the internal region, which is expressed as I_(C)(002)/{I_(C)(100)+I_(C)(002)+I_(C)(101)+I_(C)(102)+I_(C)(110)+I_(C)(103)}, is defined as a basal plane peak ratio O_(C), a ratio O_(F)/O_(C) of the basal plane peak ratio O_(F) in the surface region to the basal plane peak ratio O_(C) in the internal region satisfies 0.95≦O_(F)/O_(C)≦1.05.
 7. The magnesium alloy material according to claim 6, wherein, when an average crystal grain size in the surface region is defined as D_(F) and an average crystal grain size in the internal region is defined as D_(C), a ratio D_(C)/D_(F) of the average crystal grain size D_(C) in the internal region to the average crystal grain size D_(F) in the surface region satisfies ⅔≦D_(C)/D_(F)≦3/2≧D_(F)≧3.5 μm, and D_(C)≧3.5 μm.
 8. The magnesium alloy material according to claim 6, wherein, when a Vickers hardness (Hv) in the surface region is defined as H_(F) and a Vickers hardness (Hv) in the internal region is defined as H_(C), a ratio H_(C)/H_(F) of the Vickers hardness H_(C) in the internal region to the Vickers hardness H_(F) in the surface region satisfies 0.85≦H_(C)/H_(F)≧1.2.
 9. The magnesium alloy material according to claim 7, wherein, when a Vickers hardness (Hv) in the surface region is defined as H_(F) and a Vickers hardness (Hv) in the internal region is defined as H_(C), a ratio H_(C)/H_(F) of the Vickers hardness H_(C) in the internal region to the Vickers hardness H_(F) in the surface region satisfies 0.85≦H_(C)/H_(F)≦1.2.
 10. The magnesium alloy material according to claim 6, wherein the magnesium alloy contains Al as an additive element in a content of 5.0 mass % or more and 12 mass % or less.
 11. The magnesium alloy material according to claim 7, wherein the magnesium alloy contains Al as an additive element in a content of 5.0 mass % or more and 12 mass % or less.
 12. The magnesium alloy material according to claim 8, wherein the magnesium alloy contains Al as an additive element in a content of 5.0 mass % or more and 12 mass % or less.
 13. The magnesium alloy material according to claim 9, wherein the magnesium alloy contains Al as an additive element in a content of 5.0 mass % or more and 12 mass % or less.
 14. A method for producing a magnesium alloy material by rolling a raw material composed of a magnesium alloy, the method comprising: a preparation step of preparing a sheet-shaped raw material obtained by subjecting a molten magnesium alloy to continuous casting by a twin-roll casting process; and a rolling step of rolling the raw material with multiple passes to produce a sheet-shaped magnesium alloy material having a thickness of 1.5 mm or more, wherein, in the rolling step, at least one pass of the rolling is performed at a reduction ratio of 25% or more per pass and the remaining passes of the rolling are performed at a reduction ratio of 10% or more per pass. 